Beginning with the release of Office 2019, OneNote for Windows 10 will be the default OneNote experience for Office 365 and Office 2019. By focusing our efforts on a single Windows app, we’ll be able to bring you the very best experience and reduce any confusion about which app to use.

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What’s happening to OneNote 2016?

While we’re no longer adding new features to OneNote 2016, it’ll still be there if you need it. OneNote 2016 is optionally available for anyone with Office 365 or Office 2019, but it will no longer be installed by default. If you currently use OneNote 2016, you won’t notice any changes when you update to Office 2019. We’ll continue to offer support, bug fixes, and security updates for OneNote 2016 for the duration of the Office 2016 support lifecycle, which runs through October 2020 for mainstream support and October 2025 for extended support. In other words, you'll still receive critical fixes—but not new features—for at least the next two years. For more details, please refer to this FAQ.

There is much, much more information in the referenced article especially about the "Top-requested features coming soon to OneNote for Windows 10".

Also see the topic here as well as these additional OneNote References: